This invention relates to methods and apparatus for the use of eddy currents to detect subsurface discontinuities in metal objects. This invention is particularly directed to methods and apparatus for detecting cracks and flaws within multilayer conductive structure that is joined together by fasteners that extend through each structural layer.
Copending U.S. patent application Ser. No. 974,356, which was filed Dec. 29, 1978 and is assigned to the same assignee as this application (now U.S. Pat. No. 4,271,393), discloses nondestructive inspection of conductive bodies and structure wherein eddy currents are induced in the conductive structure by means of an excitation coil that is placed in abutment with the structure to be examined and driven with a current pulse that increases at a relatively slow rate and, upon reaching a predetermined level, decreases at a relatively rapid rate. Driven in such a manner, the excitation coil supplies a corresponding aperiodic magnetic field that penetrates the conductive material being examined during the relatively slow build-up period and produces substantial eddy currents throughout the affected conductive region as the field rapidly collapses.
To detect subsurface discontinuities, including cracks and other flaws, the composite magnetic field (which results from the interaction of the energization field and the magnetic field produced by the induced eddy currents) is detected by a sensing coil (or other magnetic flux detector) and the resulting electrical signal is compared with a signal obtained when an identically configured, flaw-free structure (i.e., a reference specimen) is electromagnetically excited in the same manner. In particular, the referenced patent application dicloses the use of a storage-type oscilloscope for simultaneous display of the electrical signal produced by the sensing coil when eddy currents are produced within the flaw free reference structure and the signal produced when eddy currents are induced in the structure being inspected.
One important aspect of eddy current inspection apparatus of the type disclosed in the above-referenced patent application is that the initial portion of the signal supplied by the sensing coil during each inspection sequence is primarily determined by structural features and discontinuities that lie in or near the surface of the conductive region being examined whereas later portions of the signal are more indicative of discontinuities that lie relatively deep within the conductive material. Accordingly, by disregarding or eliminating initial portions of the signal supplied by the sensing coil, an eddy current inspection system of the type disclosed in the above-referenced patent application can be made relatively insensitive to discontinuities in the surface of the material being inspected, including design features such as the edge of the top conductive member in a multilayer structure as well as machined apertures such as countersink and counterbore regions.
The above-discussed eddy current inspection technique provides several other advantages over previously employed eddy current inspection arrangements, which generally employ a continuous-wave (CW) electrical signal to produce the time-varying magnetic field that induces eddy currents within the material being examined. For example, as is discussed in more detail in the previously-referenced patent application, improvement of approximately one order of magnitude is obtained relative to system sensitivity or resolution in that cracks on the order of 0.2 inches in length that lie as deep as 0.5 inches below the surface of the material can be detected by utilizing an aperiodic energization signal in the disclosed manner. Further, since such a technique results in substantially lower power dissipation within the excitation coil, relatively small excitation coils can be utilized and thermal drift often encountered in systems employing a CW excitation current is eliminated. Since smaller coils can be used, a system of this type can also examine a smaller region of a conductive structure and, hence, provide improved resolution as to determining both the position and size of a subsurface crack or flaw.
Because of these and other advantages, the method and apparatus disclosed in the referenced U.S. patent application is especially advantageous in the more demanding situations such as those which require the detection of subsurface flaws in conductive material which includes various intended surface features. For example, the disclosed embodiments of the referenced patent application are configured for the inspection of multilayer conductive structure that is joined together by the use of various conventional fasteners such as rivets or bolts, with assemblies such as those utilized in modern high speed aircraft being of special significance. In this regard, in the manufacture and maintenance of aircraft and other structural assemblies that are subjected to substantial mechanical stress or strain through vibration or other physical forces, it is often necessary and desirable to detect fatigue-induced cracks or flaws that can develop in a subsurface layer, especially along the periphery of a fastener which joins such a subsurface layer to other conductive layers or various frame members.
Although offering significant advantages over prior art eddy current inspection apparatus such as those employing CW excitation signals, the method and apparatus disclosed in our previously-referenced patent application do not completely overcome all of the problems associated with the inspection of multilayer conductive structure such as that employed in aircraft and other important arrangements which present a complex electromagnetic environment that varies from one test situation to the next. In this regard, the exact nature of the induced eddy currents and hence the electrical signal supplied by the sensing coil of an eddy current inspection apparatus not only depends on the location and configuration of subsurface discontinuities such as cracks or other flaws, but is significantly affected by the structural arrangement of interest; by the magnetic and electrical properties of the materials involved; and by other factors such as the position of the excitation and sensing coils relative to the structure being examined. More specifically, even though the presence of subsurface flaws and discontinuities is determined by comparing the electrical signal produced by the sensing coil when eddy currents are produced in a flaw-free reference structure with the signal produced when an identical excitation signal is used to induce eddy currents in the structure being inspected, variations in structural arrangement and material properties that are acceptable from a manufacturing standpoint can produce fairly significant differences in the characteristics of the signal provided the sensing coil, including changes in maximum signal amplitude and the relative time between initiation of the excitation signal and the time at which the signal reaches maximum amplitude. For example, in inspecting regions of a multilayer structure that surround fasteners for fatigue-induced cracks, allowable deviation of the inspected arrangement from design value with respect to edge margin, fastener countersink configuration (including the flushness of the fastener head relative to the surface of the structure), variation in the thickness of each conductive layer and variation in the conductivity and permeability of either the fastener or one or more of the conductive layers affect the induced eddy currents and hence cause variation in the test results. Moreover, factors that are not generally controlled during the manufacture of aircraft and other such structure can substantially alter the electromagnetic environment presented by the structure being inspected and thus alter the signal supplied by the sensing coil. For example, it is common practice to coat various portions of aircraft with paint or other protective material. Since such coatings are generally nonconductive, the excitation coil is, in effect, electromagnetically spaced apart from the structure being examined, even though physical contact is maintained between the coil and the coated surface of the structure being examined. In addition, a test set operator may fail to maintain the excitation coil in full abutment with the structure under test during the inspection routine and hence introduce additional variation in the electromagnetic configuration relative to that presented by the reference specimen. Because the thickness and dielectric constant of a protective coating such as paint is not generally controlled during manufacture or maintained relatively constant during the lifetime of an aircraft or other structure the reference structure cannot generally be configured to present an electromagnetic environment substantially identical to that of the inspected conductive region.
Each factor that affects the electromagnetic characteristics of the structure being examined and varies as a function of manufacturing tolerances and material properties or as a function of uncontrolled parameters (e.g., the thickness of a protective surface coating or the positioning of the test apparatus coils) in effect decreases inspection system sensitivity and resolution relative to that which can be attained under optimal conditions. Thus, although the methods and apparatus of our previously referenced patent application provide substantial advantages over prior art CW eddy current inspection techniques, it has still been necessary to rely on the expertise of the system operator to properly interpret and analyze some of the inspection signals so as to differentiate between acceptable variations in one or more of the inspection parameters and the presence of a subsurface crack or flaw.
In addition to the fact that the method and apparatus disclosed in our above-referenced patent application do not provide optimum results relative to the information contained in the signal provided by the system sensing coil, the embodiments of eddy current inspection apparatus disclosed therein are not ideally arranged relative to use in typical production and maintenance shop environments or by personnel of typical training and ability. For example, the use of a storage-type oscilloscope for displaying the signal supplied by the sensing coil when eddy currents are produced within a flaw-free reference specimen and the signal produced when eddy currents are induced in the structure being inspected may be satisfactory when highly trained technical personnel are practicing the invention in a test laboratory, but may be unacceptable when production or maintenance personnel are attempting to locate subsurface cracks and flaws in aircraft that are located in a factory or on a flight line. Further, modern aircraft and other similar structure include an extremely large number of fastener configurations which vary as to fastener type and size as well as to the physical arrangement and properties of the joined-together conductive structure. Thus, even if eddy current inspection is utilized only with respect to those fasteners that are located in portions of the aircraft that are exposed to substantial vibration or other forces practicing the eddy current inspection scheme in the manner disclosed in our copending patent application can require a large number of reference specimens which must be transported to the inspection site and stored when not in use.
Accordingly, it is an object of this invention to provide methods and apparatus for eddy current inspection of conductive structure wherein compensation is effected relative to selected spatial, structural, and/or material characteristics that have previously affected the results of such inspection techniques.
It is another object of this invention to provide improvements in the eddy current inspection methods and apparatus disclosed in the previously-referenced copending patent application; such improvements providing improved interpretation of inspection signals provided to thereby at least partially eliminate test uncertainties resulting from otherwise acceptable variations in various parameters that affect electrical and magnetic properties of the structure being inspected.
It is still another object of this invention to provide methods and apparatus for eddy current detection of subsurface cracks and flaws in multilayer conductive structure such as the structure employed in aircraft with compensation being provided for the effect of one or more factors that affect the electromagnetic characteristics of the structure.
It is yet another object of this invention to provide methods and apparatus of the above-described nature for detecting fatigue-induced cracks about the periphery of fasteners that join one or more conductive layers or panels to additional panels or frame members with compensation being provided relative to uncontrolled variations in the spatial relationship between the test apparatus and the structure under test; such variations resulting from dielectric coatings or other factors.
Even further, it is an object of this invention to provide eddy current apparatus of the above-mentioned type which is configured and arranged to facilitate inspection of a relatively large number of fastener regions and is adapted for use by production and maintenance personnel in typical production and maintenance environments.